This invention relates generally to an apparatus and method for photochemical vapor deposition of materials on substrates and particularly to photochemical vapor deposition of semiconductor thin films.
Photochemical vapor deposition is useful for depositing thin films of conductors, insulators and semiconductors. U.S. Pat. No. 4,371,587 issued Feb. 1, 1983 to J. W. Peters describes photochemical deposition of silicon dioxide passivation layers for semiconductor devices. U.S. Pat. No. 4,265,932 issued May 5, 1981 to J. W. Peters and F. L. Gebhard describes photochemical vapor deposition of silicon nitride passivation layers for semiconductor devices. T. Inoue et al in "Photochemical Vapor Deposition of Undoped and n-Type Amorphous Silicon Films Produced from Disilane", Applied Physics Letters Volume 43, No. 3, Oct. 15, 1983, pp. 774-776, describe photochemical vapor deposition of hydrogenated amorphous silicon films for photovoltaic solar cells and thin film semiconductor devices.
The photochemical vapor deposition process is initiated by transmission of radiation through a window which forms the top of the deposition chamber. During the deposition, the window is subject to becoming coated with deposited material. This coating diminishes the transparency of the window, which diminishes the amount of radiation which enters the deposition camber to initiate the photochemical reaction, and thus impedes the rate of deposition and the ultimate thickness of deposition that may be achieved.
Several solutions to the problem of deposition on the window have been proposed. In the aforementioned publication by Inoue et al, coating of the window with a low vapor pressure oil in order to prevent the window from clouding due to deposition of amorphous silicon film on it was described.
T. Kazahaya et al in "Valency Control in Photochemically-Deposited a-Si:H from Si.sub.2 H.sub.6 "Technical Digest of the International PVSEC-1, Kobe, Japan (1984), pp. 449-552 describe blowing a stream of nitrogen gas near the window. A similar approach is described in U.S. Pat. No. 4,435,445 issued Mar. 6, 1984 to D. D. Allred et al.
The aforementioned U.S. Pat. No. 4,265,932 discloses a mobile transparent window apparatus comprising a polyvinylidene fluoride film that is drawn across the internal face of the window within the reaction chamber and means for moving the film across the internal face of the window. The mobile film is placed at the internal face of the reactor window during deposition in order to prevent undesirable deposition of material on the internal face of the reactor window. Movement of the film across the internal face of the reactor window is provided to remove material deposited on the film from the path of incoming radiation and to maintain the window in a clean and transparent state. Guide rolls are provided for optimal spacing, including contact, between the mobile film and window but with minimized friction between the window and film so that reactant gases are kept out of contact with the internal face of the window.
None of these solutions have proven to be entirely satisfactory. Coating the reactor window with oil, as described in the Inoue et al publication, introduces contamination, is not reproducible and is not effective for long periods of time. Providing a stream of non-reactive gas across the face of the reactor window, as described in the Kazahaya et al publication and U.S. Pat. No. 4,435,445, has limited effectiveness due to unavoidable transport of reactive gses to the region of the reactor window. The mobile transparent window of U.S. Pat. No. 4,265,932 requires a mechanically imposed gas-tight seal between the film and reactor window. Leakage of reactive gas into the space between the film and window is difficult to avoid during long periods of operation.